Power supply system for electrically powered vehicle, electrically powered vehicle, and method for controlling the same

ABSTRACT

A power supply system includes a main power storage device and a plurality of sub power storage devices. A converter is connected to selected one of the sub power storage devices to convert voltage between the selected sub power storage device and an electric power feeding line bidirectionally. When the travel mode of an electrically powered vehicle is an EV mode, switching processing of a sub power storage device is performed based on the selected sub power storage device&#39;s SOC. On the other hand, when the travel mode of the electrically powered vehicle is an HV mode, control under which the SOC of the main power storage device and the plurality of sub power storage devices as a whole is kept constant is carried out, and switching of the selected sub power storage device is prohibited.

CROSS REFERENCE TO RELATED APPLICATION

This application is a National Stage of International Application No.PCT/JP2008/069871 filed Oct. 31, 2008, the contents of all of which areincorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a power supply system for anelectrically powered vehicle and a method for controlling the same, andmore particularly to control of a power supply system of an electricallypowered vehicle having a main power storage device and a plurality ofsub power storage devices mounted thereon.

BACKGROUND ART

In recent years, as an environmentally friendly vehicle, electricallypowered vehicles such as electric cars, hybrid cars and fuel cell carshave been developed into practical use. These electrically poweredvehicles have mounted thereon an electric motor generating force todrive the vehicle and a power supply system for supplying electric powerto drive the motor, that is configured to include a power storagedevice.

In particular for hybrid cars, there has been proposed a configurationcharging a vehicle-mounted power storage device by a power supplyexternal to the vehicle (hereinafter also referred to as an “externalpower supply”), and accordingly, these electrically powered vehiclesrequire increased distances travelable on electric power stored in thevehicle-mounted power storage device. Hereinafter, charging avehicle-mounted power storage device by an external power supply willalso simply be referred to as “external charging”.

For example, Japanese Patent Laying-Open No. 2008-109840 (PatentDocument 1) and Japanese Patent Laying-Open No. 2003-209969 (PatentDocument 2) describe a power supply system having a plurality of powerstorage devices (batteries) connected in parallel. The power supplysystem described in Patent Documents 1 and 2 is provided with a voltageconverter (a converter) for each power storage device (battery) as acharging/discharging adjustment mechanism. In contrast, Japanese PatentLaying-Open No. 2008-167620 (Patent Document 3) describes aconfiguration of a power supply device in a vehicle having a main powerstorage device and a plurality of sub power storage devices mountedthereon, that provides a converter associated with a main power supplydevice and a converter shared by the plurality of sub power storagedevices. This configuration can achieve a reduced number of elements inthe device and also an increased storable amount of energy.

Furthermore, Japanese Patent Laying-Open No. 2006-77641 (Patent Document4) describes a series/parallel hybrid electric car which has mountedthereon a control device for switching the driving state of a vehiclebased on the vehicle speed and an SOC. The driving state includes astate in which an output from an engine (internal combustion engine) isused as motive power for traveling and a state in which motive power fortraveling is obtained by stopping the engine and operating a motor.

-   Patent Document 1: Japanese Patent Laying-Open No. 2008-109840-   Patent Document 2: Japanese Patent Laying-Open No. 2003-209969-   Patent Document 3: Japanese Patent Laying-Open No. 2008-167620-   Patent Document 4: Japanese Patent Laying-Open No. 2006-77641

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In the power supply device described in Patent Document 3, one of theplurality of sub power storage devices is selectively connected to theconverter to allow the main power supply device and the selected subpower storage device to supply electric power to drive an electric motorfor driving a vehicle. In such a power supply device, when the sub powerstorage device in use has a decreased SOC, a different sub power storagedevice is connected to the converter to use the plurality of sub powerstorage devices sequentially to allow stored electric energy to be usedto achieve increased electric vehicle (EV) travelable distance.

The spent sub power storage device has a low SOC value, and therefore isconsidered to be non-reconnectable to the converter during traveling ofthe vehicle. That is, switching of a sub power storage device to beconnected to the converter during traveling of the electrically poweredvehicle certainly decreases stored electric energy available for theelectrically powered vehicle, which influences on EV traveling. Forexample, EV traveling may not be carried out (for instance, an enginemay start in a case of a hybrid car) despite the fact that a user wantsthe electrically powered vehicle to carry out EV traveling. In order toavoid such a problem, it is necessary to appropriately perform aconnection switching process for changing a sub power storage device tobe used.

The present invention has been made to solve such problems and an objectof the present invention is to appropriately perform a connectionswitching process for changing a sub power storage device to be used, ina power supply system configured to include a main power storage deviceand a plurality of sub power storage devices sharing a voltage converter(a converter).

Means for Solving the Problems

According to one aspect of the present invention, a power supply systemfor an electrically powered vehicle incorporating a motor for generatingpower to drive the vehicle includes a main power storage device, anelectric power feeding line, a first voltage converter, a plurality ofsub power storage devices provided in parallel to each other, a secondvoltage converter, a connection unit, and a switching control device.The electric power feeding line feeds electric power to an inverter fordriving and controlling the motor. The first voltage converter isprovided between the electric power feeding line and the main powerstorage device, and configured to convert voltage therebetweenbidirectionally. The second voltage converter is provided between theplurality of sub power storage devices and the electric power feedingline, and configured to convert voltage between one of the plurality ofsub power storage devices and the electric power feeding linebidirectionally. The connection unit is provided between the pluralityof sub power storage devices and the second voltage converter, andconfigured to selectively connect a sub power storage device selectedfrom the plurality of sub power storage devices to the second voltageconverter. The switching control device is configured to controlselective connection between the plurality of sub power storage devicesand the second voltage converter. The switching control device includesa switching determination unit and a connection switching unit. Theswitching determination unit determines whether the selected sub powerstorage device should be switched based on a state of charge of each ofthe plurality of sub power storage devices. The connection switchingunit is configured to switch connection between the plurality of subpower storage devices and the second voltage converter when theswitching determination unit determines that the selected sub powerstorage device should be switched. The switching determination unitdetermines that the selected sub power storage device should not beswitched regardless of the state of charge of the selected sub powerstorage device, when receiving a switching prohibit instruction.

Preferably, the electrically powered vehicle includes an internalcombustion engine configured to be able to generate the power to drivethe vehicle independently of the motor and a charging and dischargingcontrol unit. The charging and discharging control unit sets a travelmode of the electrically powered vehicle based on a total required powerof the electrically powered vehicle either in a first mode allowing themotor to generate the power to drive the vehicle or in a second modeallowing the motor and the internal combustion engine to generate thepower to drive the vehicle. The charging and discharging control unitcontrols charging and discharging to and from the main power storagedevice and the plurality of sub power storage devices in accordance withthe travel mode set. The switching determination unit receives theswitching prohibit instruction from the charging and discharging controlunit while the travel mode is the second mode.

Preferably, remaining capacity of the main power storage device and theplurality of sub power storage devices as a whole is controlled by thecharging and discharging control unit to be kept constant in the secondmode.

According to another aspect of the present invention, an electricallypowered vehicle includes a motor, an inverter, an electric power feedingline, a main power storage device, a first voltage converter, aplurality of sub power storage devices provided in parallel to eachother, a second voltage converter, a connection unit, and a controldevice. The motor generates power to drive the vehicle. The inverterdrives and controls the motor. The electric power feeding line feedselectric power to the inverter. The first voltage converter is providedbetween the electric power feeding line and the main power storagedevice, and configured to convert voltage therebetween bidirectionally.The second voltage converter is provided between the plurality of subpower storage devices and the electric power feeding line, andconfigured to convert voltage between one of the plurality of sub powerstorage devices and the electric power feeding line bidirectionally. Theconnection unit is provided between the plurality of sub power storagedevices and the second voltage converter, and configured to selectivelyconnect a sub power storage device selected from the plurality of subpower storage devices to the second voltage converter. The controldevice controls at least the connection unit. The control deviceincludes a switching determination unit and a connection switching unit.The switching determination unit determines whether the selected subpower storage device should be switched based on a state of charge ofeach of the plurality of sub power storage devices. The connectionswitching unit is configured to switch connection between the pluralityof sub power storage devices and the second voltage converter when theswitching determination unit determines that the selected sub powerstorage device should be switched. The switching determination unitdetermines that the selected sub power storage device should not beswitched regardless of the state of charge of the selected sub powerstorage device, when receiving a switching prohibit instruction.

Preferably, the electrically powered vehicle further includes aninternal combustion engine configured to be able to generate the powerto drive the vehicle independently of the motor. The control devicefurther includes a charging and discharging control unit. The chargingand discharging control unit sets a travel mode of the electricallypowered vehicle based on a total required power of the electricallypowered vehicle either in a first mode allowing the motor to generatethe power to drive the vehicle or in a second mode allowing the motorand the internal combustion engine to generate the power to drive thevehicle. The charging and discharging control unit controls charging anddischarging to and from the main power storage device and the pluralityof sub power storage devices in accordance with the travel mode set. Thecharging and discharging control unit generates the switching prohibitinstruction while the travel mode is the second mode, and stopsgeneration of the switching prohibit instruction while the travel modeis the first mode.

Preferably, the charging and discharging control unit controls theinverter and the internal combustion engine such that remaining capacityof the main power storage device and the plurality of sub power storagedevices as a whole is kept constant in the second mode.

Preferably, the electrically powered vehicle further includes a travelmode setting device. The travel mode setting device has a first stateand a second state corresponding to the first and second modes,respectively, and either one of the first and second states isconfigured to be manually settable. The charging and discharging controlunit sets the travel mode based on the one state in the travel modesetting device.

According to still another aspect of the present invention, a method forcontrolling an electrically powered vehicle is provided The electricallypowered vehicle includes a motor, an inverter, an electric power feedingline, a main power storage device, a first voltage converter, aplurality of sub power storage devices provided in parallel to eachother, a second voltage converter, a connection unit, and a controldevice. The motor generates power to drive the vehicle. The inverterdrives and controls the motor. The electric power feeding line feedselectric power to the inverter. The first voltage converter is providedbetween the electric power feeding line and the main power storagedevice, and configured to convert voltage therebetween bidirectionally.The second voltage converter is provided between the plurality of subpower storage devices and the electric power feeding line, andconfigured to convert voltage between one of the plurality of sub powerstorage devices and the electric power feeding line bidirectionally. Theconnection unit is provided between the plurality of sub power storagedevices and the second voltage converter, and configured to selectivelyconnect a sub power storage device selected from the plurality of subpower storage devices to the second voltage converter. The controldevice controls at least the connection unit. The method includes thesteps of determining whether the selected sub power storage deviceshould be switched based on a state of charge of each of the pluralityof sub power storage devices and switching connection between theplurality of sub power storage devices and the second voltage converterwhen the step of determining determines that the selected sub powerstorage device should be switched. The step of determining determinesthat the selected sub power storage device should not be switchedregardless of the state of charge of the selected sub power storagedevice, when switching of the selected sub power storage device isprohibited.

Preferably, the electrically powered vehicle further includes aninternal combustion engine configured to be able to generate the powerto drive the vehicle independently of the motor. The method furtherincludes the steps of setting a travel mode of the electrically poweredvehicle either in a first mode allowing the motor to generate the powerto drive the vehicle or in a second mode allowing the motor and theinternal combustion engine to generate the power to drive the vehicle,controlling charging and discharging to and from the main power storagedevice and the plurality of sub power storage devices in accordance withthe travel mode set, and prohibiting switching of the selected sub powerstorage device when the step of setting sets the travel mode in thesecond mode.

Preferably, the step of controlling controls the inverter and theinternal combustion engine such that remaining capacity of the mainpower storage device and the plurality of sub power storage devices as awhole is kept constant in the second mode.

Effects of the Invention

According to the present invention, in the power supply systemconfigured to include a main power storage device and a plurality of subpower storage devices, with the plurality of sub power storage devicessharing a voltage converter (a converter), the connection switchingprocess for changing a sub power storage device to be used canappropriately be performed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a main configuration of an electricallypowered vehicle incorporating a power supply system according to anembodiment of the present invention.

FIG. 2 is a circuit diagram showing in detail a configuration ofinverters 14 and 22 shown in FIG. 1.

FIG. 3 is a circuit diagram showing in detail a configuration ofconverters 12A and 12B shown in FIG. 1.

FIG. 4 is a functional block diagram of a control device 30.

FIG. 5 is a functional block diagram for illustrating acharging/discharging control unit 52.

FIG. 6 is a flowchart for illustrating a flag FLG setting process by atraveling control unit 250.

FIG. 7 is a flowchart for illustrating a charging/discharging controlprocess by charging/discharging control unit 52.

FIG. 8 is a flowchart of a general procedure of a process for switchinga selected sub power storage device in the power supply system of theelectrically powered vehicle according to an embodiment of the presentinvention.

FIG. 9 is a flowchart for illustrating in detail the process in FIG. 8performed to determine whether the selected sub power storage deviceshould be switched or not (S100).

FIG. 10 is a flowchart for illustrating in detail a pre-switchingvoltage step-up process (S200) shown in FIG. 8.

FIG. 11 is a flowchart for illustrating in detail an electric powerlimit modification process (S300) shown in FIG. 8.

FIG. 12 is a flowchart for illustrating in detail a process forswitching connection of the sub power storage device (S400), as shown inFIG. 8.

FIG. 13 is a flowchart for illustrating in detail a return process(S500) shown in FIG. 8.

FIG. 14 is an operation waveform diagram in the process for switching aselected sub power storage device in the power supply system of theelectrically powered vehicle according to an embodiment of the presentinvention.

FIG. 15 is a functional block diagram for illustrating a configurationof a switching control unit 51.

FIG. 16 illustrates control of batteries' SOC in an HV mode.

FIG. 17 illustrates a state in which the total SOC has lowered whileelectrically powered vehicle 1 is traveling in an HV mode.

DESCRIPTION OF THE REFERENCE SIGNS

1; 2 wheel; 3 power split device; 4 engine; 6 battery charging converter(external charging); 8 external power supply; 9A, 9B1, 9B2 currentsensor; 10A, 10B1, 10B2, 13, 21A, 21B voltage sensor; 11A, 11B1, 11B2temperature sensor; 12A converter (dedicated to main power storagedevice); 12B converter (shared by sub power storage devices); 14, 22inverter; 15 to 17 each phase arm (U, V, W); 24, 25 current sensor; 30control device; 39A connection unit (for main power storage device); 39Bconnection unit (for sub power storage device); 40 EV switch; 51switching control unit; 52 charging/discharging control unit; 100switching determination unit; 110 step-up-voltage instruction unit; 120electric power limiter unit (for main power storage device); 130electric power limiter unit (for sub power storage device); 140connection control unit; 200 converter control unit; 250 travelingcontrol unit; 260 total power calculation unit; 270, 280 invertercontrol unit; BA battery (main power storage device); BB selected subpower storage device; BB1, BB2 battery (sub power storage device); C1,C2, CH smoothing capacitor; CMBT step-up voltage command signal; CONT1to CONT7 relay control signal; D1 to D8 diode; FBT flag (stepping upvoltage completed); FLG flag (switching prohibit instruction); IA, IB1,1B2 input/output current (battery); ID variable (status of switchingprocess); IGON start signal; L1 reactor; MCRT1, MCRT2 motor currentvalue; MG1, MG2 motor-generator; N2 node; PL1A, PL1B power supply line;PL2 electric power feeding line; Pttl total required power; PWMI, PWMI1,PWMI2, PWMC, PWMC1, PWMC2 control signal (for inverter); PWU, PWUA,PWDA, PWD, PWDA, PWDB control signal (for converter); Q1 to Q8 IGBTdevice; R limiting resistor; SL1, SL2 ground line; SMR1 to SMR3 systemmain relay; SR1, SR1G, SR2, SR2G relay; TA, TBB1, TBB2 batterytemperature (battery); Tqcom1, Tqcom2 torque command value; UL, VL, WLline (three-phase); V1 predetermined voltage; VBA, VBB1, VBB2 voltage(battery output voltage); VLA, VLB, VH voltage; VHref voltage commandvalue (VH); Win upper limit on electric power input; Win(M) upper limiton electric power input (to main power storage device); Win(S) upperlimit on electric power input (to selected sub power storage device);Wout upper limit on electric power output; Wout(M) upper limit onelectric power output (from main power storage device); and Wout(S)upper limit on electric power output (from selected sub power storagedevice).

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter reference will be made to the drawings to more specificallydescribe the present invention in embodiments. In the followingdescription, identical or equivalent components in the drawings aredenoted by identical reference characters and will in principle not bedescribed repeatedly.

FIG. 1 shows a main configuration of an electrically powered vehicleincorporating a power supply system according to an embodiment of thepresent invention.

With reference to FIG. 1, an electrically powered vehicle 1 includespower storage devices implemented as batteries BA, BB1, BB2, connectionunits 39A, 39B, converters 12A, 12B, smoothing capacitors C1, C2, CH,voltage sensors 10A, 10B1, 10B2, 13, 21A, 21B, temperature sensors 11A,11B1, 11B2, current sensors 9A, 9B1, 9B2, an electric power feeding linePL2, inverters 14, 22, motor-generators MG1, MG2, a wheel 2, a powersplit device 3, an engine 4, and a control device 30.

The present embodiment provides a power supply system of theelectrically powered vehicle including a main power storage deviceimplemented as battery BA, electric power feeding line PL2 supplyingelectric power to inverter 14 driving motor-generator MG2, converter 12Aprovided between main power storage device (BA) and electric powerfeeding line PL2 to serve as a voltage converter converting voltagebidirectionally, batteries BB1, BB2 implementing a plurality of subpower storage devices provided in a manner parallel to each other, andconverter 12B provided between the plurality of sub power storagedevices (BB1, BB2) and electric power feeding line PL2 to serve as avoltage converter converting voltage bidirectionally. Voltage converter(12B) is connected selectively to one of the plurality of sub powerstorage devices (BB1, BB2) to convert voltage between the connected subpower storage device and electric power feeding line PL2bidirectionally.

A sub power storage device (one of BB1 and BB2) and the main powerstorage device (BA) have their storable capacity set so that, forexample, when they are concurrently used, they can output maximum powertolerated for an electric load (22 and MG2) connected to the electricpower feeding line. This allows the vehicle without using the engine,i.e., traveling as an EV (Electric Vehicle), to travel with maximumpower. If the sub power storage device's power storage state becomeslower, the sub power storage device can be exchanged to cause thevehicle to further travel, and if the sub power storage device'selectric power has completely been consumed, then, in addition to themain power storage device, the engine can be used to allow the vehicleto travel with maximum power without using the sub power storage device.

Furthermore, such a configuration allows converter 12B to be sharedbetween the plurality of sub power storage devices. This can eliminatethe necessity of increasing the number of converters to be equal to thatof power storage devices. For further increased EV travelable distance,an additional battery can be introduced in parallel with batteries BB1,BB2.

Preferably, this electrically powered vehicle has mounted thereon a mainpower storage device and sub power storage devices that are externallychargeable. For this purpose, electrically powered vehicle 1 furtherincludes a battery charging device (a charging converter) 6 forconnection to an external power supply 8 which is for example acommercial power supply of AC 100V. Battery charging device (6) convertsalternate current to direct current and also adjusts voltage to supplyelectric power charged to a battery. Note that external charging may beachieved by the above-described configuration and in addition a systemconnecting a neutral point of a stator coil of motor-generator MG1, MG2to alternate current power supply or a system causing converters 12A,12B to together function as an AC/DC conversion device.

Smoothing capacitor C1 is connected between a power supply line PL1A anda ground line SL2. Voltage sensor 21A detects a voltage VLA across endsof smoothing capacitor C1 and outputs it to control device 30. Converter12A can step up voltage across terminals of smoothing capacitor C1 andsupply it to electric power feeding line PL2.

Smoothing capacitor C2 is connected between a power supply line PL1B andground line SL2. Voltage sensor 21B detects a voltage VLB across ends ofsmoothing capacitor C2 and outputs it to control device 30. Converter12B can step up voltage across terminals of smoothing capacitor C2 andsupply it to electric power feeding line PL2.

Smoothing capacitor CH smoothes the voltage stepped up by converter 12A,12B. Voltage sensor 13 senses a voltage VH across terminals of smoothingcapacitor CH and outputs it to control device 30.

Alternatively, in an opposite direction, converters 12A, 12B can stepdown voltage VH across terminals smoothed by smoothing capacitor CH andsupply it to power supply lines PL1A, PL1B.

Inverter 14 receives direct current voltage from converter 12B and/or12A, converts it to three-phase alternate current voltage, and outputsit to motor-generator MG1. Inverter 22 receives direct current voltagefrom converter 12B and/or 12A, converts it to three-phase alternatecurrent voltage, and outputs it to motor-generator MG2.

Power split device 3 is a mechanism coupled to engine 4 andmotor-generators MG1, MG2 to distribute motive power therebetween. Thepower split device can for example be a planetary gear mechanism havingthree shafts of rotation of a sun gear, a planetary carrier, and a ringgear. In the planetary gear mechanism, when two of the three shafts ofrotation have their rotation determined, that of the other one shaft ofrotation is compulsively determined. These three shafts of rotation areconnected to engine 4 and motor-generators MG1, MG2 at their respectiveshafts of rotation, respectively. Motor-generator MG2 has its shaft ofrotation coupled to wheel 2 by a reduction gear, a differential gear orthe like (not shown). Furthermore, power split device 3 may further havea speed reducer incorporated therein for the shaft of rotation ofmotor-generator MG2.

Connection unit 39A includes a system main relay SMR2 connected betweenthe positive electrode of battery BA and power supply line PL1A, asystem main relay SMR1 and a limiting resistor R connected in series andconnected in parallel with system main relay SMR2, and a system mainrelay SMR3 connected between the negative electrode of battery BA (aground line SL1) and a node N2.

System main relays SMR1 to SMR3 have their conduction/non-conductionstates controlled (or are turned on/off) by relay control signals CONT1to CONT3, respectively, issued from control device 30.

Voltage sensor 10A measures a voltage VA across terminals of battery BA.Furthermore, temperature sensor 11A measures a temperature TA of batteryBA, and current sensor 9A measures a current IA input/output to/frombattery BA. These sensors' measurements are output to control device 30.Based on these measurements, control device 30 monitors a state ofbattery BA represented by state of charge (SOC).

Connection unit 39B is provided between power supply line PL1B andground line SL2, and batteries BB1, BB2. Connection unit 39B includes arelay SR1 connected between the positive electrode of battery BB1 andpower supply line PL1B, a relay SR1G connected between the negativeelectrode of battery BB1 and ground line SL2, a relay SR2 connectedbetween the positive electrode of battery BB2 and power supply linePL1B, and a relay SR2G connected between the negative electrode ofbattery BB2 and ground line SL2.

Relays SR1, SR2 have their conduction/non-conduction states controlled(or are turned on/off) by relay control signals CONT4, CONT5,respectively, issued from control device 30. Relays SR1G, SR2G havetheir conduction/non-conduction states controlled (or are turned on/off)by relay control signals CONT6, CONT7, respectively, issued from controldevice 30. Ground line SL2 extends through converters 12A, 12B towardinverters 14 and 22, as will be described later.

Voltage sensors 10B1 and 10B2 measure voltages VBB1 and VBB2 acrossterminals of batteries BB1 and BB2, respectively. Temperature sensors11B1 and 11B2 measure temperatures TBB1 and TBB2 of batteries BB1 andBB2, respectively. Current sensors 9B1 and 9B2 measure currents IB1 andIB2 input/output to/from batteries BB1 and BB2, respectively. Thesesensors' measurements are output to control device 30. Based on thesemeasurements, control device 30 monitors the states of batteries BB1,BB2 represented by state of charge (SOC).

Battery BA, BB1, BB2 can for example be a lead-acid battery, a nickelmetal hydride battery, a lithium ion battery, or a similar secondarybattery, an electric double layer capacitor or a similar capacitor oflarge capacity, or the like.

Inverter 14 is connected to electric power feeding line PL2 and groundline SL2. Inverter 14 receives a voltage stepped up from converter 12Aand/or converter 12B, and drives motor-generator MG1 for example tostart engine 4. Furthermore, inverter 14 returns to converters 12A and12B the electric power generated by motor-generator MG1 by motive powertransmitted from engine 4. At this time, converters 12A and 12B arecontrolled by control device 30 to operate as step-down converters.

Current sensor 24 detects a current that flows to motor-generator MG1 asa motor current value MCRT1, and outputs motor current value MCRT1 tocontrol device 30.

Inverter 22 is connected to electric power feeding line PL2 and groundline SL2 in a manner parallel with inverter 14. Inverter 22 receivesdirect current voltage from converters 12A and 12B, converts it tothree-phase alternate current voltage, and outputs it to motor-generatorMG2 driving wheel 2. Furthermore, inverter 22 returns to converters 12Aand 12B the electric power generated by motor-generator MG2 as thevehicle is regeneratively braked. At this time, converters 12A and 12Bare controlled by control device 30 to operate as step-down converters.

Current sensor 25 detects a current that flows to motor-generator MG2 asa motor current value MCRT2, and outputs motor current value MCRT2 tocontrol device 30.

Control device 30 is constituted of an electronic control unit (ECU)having a central processing unit (CPU) and a memory (not shown)incorporated therein, and in accordance with a map and a program storedin the memory, uses each sensor's measurement to perform operationprocessing. Note that control device 30 may have a portion configured toallow an electronic circuit or similar hardware to perform predeterminedarithmetic and logical operations.

More specifically, control device 30 receives torque command values formotor-generators MG1, MG2, respectively, the motor-generators'respective speeds, the voltage VBA, VBB1, VBB2, VLA, VLB, VH values,motor current values MCRT1, MCRT2, and a start signal IGON. Then,control device 30 outputs a control signal PWUB instructing converter12B to step up voltage, a control signal PWDB instructing converter 12Bto step down voltage, and a shutdown signal prohibiting an operation.

Furthermore, control device 30 outputs a control signal PWMI1instructing inverter 14 to convert direct current voltage output fromconverters 12A, 12B to alternate current voltage for drivingmotor-generator MG1, and a control signal PWMC1 instructing inverter 14to convert alternate current voltage generated by motor-generator MG1 todirect current voltage and return it toward converters 12A, 12B forregeneration.

Similarly, control device 30 outputs a control signal PWMI2 instructinginverter 22 to convert direct current voltage to alternate currentvoltage for driving motor-generator MG2, and a control signal PWMC2instructing inverter 22 to convert alternate current voltage generatedby motor-generator MG2 to direct current voltage and return it towardconverters 12A, 12B for regeneration.

Electrically powered vehicle 1 further includes an EV switch 40. EVswitch 40 is operated by a user. EV switch 40 is for switching thetravel mode of electrically powered vehicle 1 between an EV mode and anHV mode. The EV mode is a mode in which battery's electric power ispositively used for driving motor-generator MG2, thereby causingmotor-generator MG2 to generate power to drive the vehicle. In the EVmode, engine 4 is basically stopped. On the other hand, the HV mode is atravel mode when the EV mode is not selected (when the EV mode iscanceled), and more specifically, a mode in which power to drive thevehicle is generated by motor-generator MG2 and engine 4.

EV switch 40 has an ON state and an OFF state. The ON state and the OFFstate correspond to the HV mode and the EV mode, respectively. Controldevice 30 detects the state of EV switch 40, thereby setting the travelmode either in the HV mode or in the EV mode.

In the present embodiment, when a user operates EV switch 40 so that ithas the ON state, the EV mode is canceled and the travel mode is set inthe HV mode. EV switch 40 may, however, have an ON state and an OFFstate that correspond to the EV mode and the HV mode, respectively.

FIG. 2 is a circuit diagram showing in detail a configuration ofinverters 14 and 22 shown in FIG. 1.

With reference to FIG. 2, inverter 14 includes a U-phase arm 15, aV-phase arm 16, and a W-phase arm 17. U-phase arm 15, V-phase arm 16,and W-phase arm 17 are connected between electric power feeding line PL2and ground line SL2 in parallel.

U-phase arm 15 includes insulated gate bipolar transistor (IGBT) devicesQ3, Q4 connected in series between electric power feeding line PL2 andground line SL2, IGBT devices Q3, Q4, and their respective anti-paralleldiodes D3, D4. Diode D3 has its cathode connected to IGBT device Q3 atthe collector, and its anode to IGBT device Q3 at the emitter. Diode D4has its cathode connected to IGBT device Q4 at the collector and itsanode to IGBT device Q4 at the emitter.

V-phase arm 16 includes IGBT devices Q5, Q6 connected in series betweenelectric power feeding line PL2 and ground line SL2, and theirrespective anti-parallel diodes D5, D6. IGBT devices Q5, Q6 andanti-parallel diodes D5, D6 are connected similarly as in U-phase arm15.

W-phase arm 17 includes IGBT devices Q7, Q8 connected in series betweenelectric power feeding line PL2 and ground line SL2, and theirrespective anti-parallel diodes D7, D8. IGBT devices Q7, Q8 andanti-parallel diodes D7, D8 are also connected similarly as in U-phasearm 15.

Note that in the present embodiment an IGBT device is indicated as arepresentative example of a power semiconductor switching elementcontrollable to be turned on/off. In other words, it is also replaceablewith a bipolar transistor, a field effect transistor, or a similar powersemiconductor switching element.

Each phase arm has an intermediate point connected to motor-generatorMG1 at each phase coil at each phase end. In other words,motor-generator MG1 is a three-phase permanent magnet synchronous motorand the three U-, V-, W-phase coils each have one end connected togetherto an intermediate point. The U-phase coil has the other end connectedto a line UL drawn from a connection node of IGBT devices Q3, Q4. TheV-phase coil has the other end connected to a line VL drawn from aconnection node of IGBT devices Q5, Q6. The W-phase coil has the otherend connected to a line WL drawn from a connection node of IGBT devicesQ7, Q8.

Inverter 22 shown in FIG. 1 is also different in that it is connected tomotor-generator MG2, however, its internal circuit configuration issimilar to inverter 14. Accordingly, it will not be described repeatedlyin detail. Furthermore, FIG. 2 shows an inverter receiving controlsignals PWMI, PWMC. This is to avoid complexity. Specifically, as shownin FIG. 1, different control signals PWMI1, PWMC1 and control signalsPWMI2, PWMC2 are input to inverters 14, 22, respectively.

FIG. 3 is a circuit diagram showing in detail a configuration ofconverters 12A and 12B shown in FIG. 1.

With reference to FIG. 3, converter 12A includes a reactor L1 having oneend connected to power supply line PL1A, IGBT devices Q1, Q2 connectedin series between electric power feeding line PL2 and ground line SL2,and their respective anti-parallel diodes D1, D2.

Reactor L1 has the other end connected to IGBT device Q1 at the emitterand to IGBT device Q2 at the collector. Diode D1 has its cathodeconnected to IGBT device Q1 at the collector and its anode to IGBTdevice Q1 at the emitter. Diode D2 has its cathode connected to IGBTdevice Q2 at the collector and its anode to IGBT device Q2 at theemitter.

Converter 12B shown in FIG. 1 is again different from converter 12A inthat the former is not connected to power supply line PL1A and insteadto power supply line PL1B, however, its internal circuit configurationis similar to converter 12A. Accordingly, it will not be describedrepeatedly in detail. Furthermore, FIG. 3 shows a converter receivingcontrol signals PWU, PWD, however, this is to avoid complexity. As shownin FIG. 1, different control signals PWUA, PWDA and control signalsPWUB, PWDB are input to inverters 14, 22, respectively.

In the power supply system of electrically powered vehicle 1, battery BA(the main power storage device) and a sub power storage device selectedfrom batteries BB1, BB2 (hereinafter also referred to as a “selected subpower storage device BB”) and motor-generators MG1, MG2 supply andreceive electric power therebetween.

Control device 30 receives values detected by voltage sensor 10A,temperature sensor 11A and current sensor 9A, and in accordancetherewith sets an SOC(BA) indicating the main power storage device'sresidual capacity, an upper limit on electric power input Win(M)indicating an upper limit value of electric power charged thereto, andan upper limit on electric power output Wout(M) indicating an upperlimit value of electric power discharged therefrom.

Furthermore, control device 30 receives values detected by voltagesensors 10B1, 10B2, temperature sensors 11B1, 11B2 and current sensors9B1, 9B2 and in accordance therewith sets an SOC(BB) of selected subpower storage device BB and upper limits on electric power input andoutput Win(S), Wout(S) thereto and therefrom, respectively.

Generally, an SOC is indicated by a ratio (%) of each battery's currentcharged amount to its fully charged state. Furthermore, Win, Wout areindicated as such an upper limit value of electric power that, when thatelectric power is discharged for a predetermined period of time (e.g.,for approximately 10 seconds), the battery of interest (BA, BB1, BB2) isnot overcharged/overdischarged.

FIG. 4 is a functional block diagram of control device 30. FIG. 4 showsfunctional blocks, which are implemented by control device 30 executinga previously stored, predetermined program and/or by processing of anoperation by electronic circuitry (hardware) in control device 30.

With reference to FIG. 4, control device 30 includes a switching controlunit 51 and a charging/discharging control unit 52. Switching controlunit 51 receives each value of voltages VH, VLA, SOC(BB1), SOC(BB2) andtemperatures TBB1, TBB2 to output signals CONT4 to CONT7 and signal PWUA(or PWDA) for switching a selected sub power storage device. Note thatcontrol of switching of a selected sub power storage device will bedescribed later in detail.

Switching control unit 51 receives a flag FLG (switching permissionflag) indicating permission for switching of a selected sub powerstorage device. Flag FLG enters an ON state when switching of a selectedsub power storage device is permitted, and flag FLG enters an OFF statewhen switching of a selected sub power storage device is prohibited.Switching control unit 51 carries out the above-described switchingcontrol when flag FLG is in the ON state. Switching control unit 51further outputs Win(M), Wout(M), Win(S), and Wout(S). In other words,flag FLG in the OFF state corresponds to an instruction for prohibitingswitching control by switching control unit 51.

Charging/discharging control unit 52 controls charging and dischargingto and from the main power storage device and the sub power storagedevices when electrically powered vehicle 1 is traveling. Specifically,charging/discharging control unit 52 controls power distribution betweenengine 4 and motor-generators MG1, MG2. For this purpose,charging/discharging control unit 52 receives motor current valuesMCRT1, MVCRT2, a request amount of regenerative braking, upper limits onelectric power input Win(M), Win(S), and upper limits on electric poweroutput Wout(M), Wout(S) to control charging and discharging to and frommain power storage device BA and sub power storage devices BB1, BB2 inaccordance with upper limits on electric power input Win(M), Win(S) orupper limits on electric power output Wout(M), Wout(S).

Charging/discharging control unit 52 sets, in response to operation ofEV switch 40 by a user, the travel mode of the electrically poweredvehicle either in the EV mode or in the HV mode. Charging/dischargingcontrol unit 52 sets flag FLG in the ON state when the travel mode isset in the EV mode. In this case, switching control unit 51 is permittedto control switching of a sub power storage device. On the other hand,when the travel mode is set in the HV mode, charging/discharging controlunit 52 sets flag FLG in the OFF state. In this case, switching controlunit 51 is prohibited from controlling switching of a sub power storagedevice.

It is noted that the fact that flag FLG enters the OFF state correspondsto the fact that charging/discharging control unit 52 has generated aswitching prohibit instruction. In addition, the fact that flag FLGenters the ON state corresponds to the fact that charging/dischargingcontrol unit 52 has stopped generating the switching prohibitinstruction.

FIG. 5 is a functional block diagram for illustratingcharging/discharging control unit 52. With reference to FIG. 5,charging/discharging control unit 52 includes a traveling control unit250, a total power calculation unit 260 and inverter control units 270,280.

Total power calculation unit 260 calculates total power Pttl requiredfor the entirety of electrically powered vehicle 1 from a vehicularspeed and an operation of a pedal (an accelerator pedal). Note thattotal required power Pttl may also include power required (i.e., theengine's output), depending on a vehicle's condition, for generatingelectric power by motor-generator MG1 to charge a battery.

Traveling control unit 250 receives upper limits on electric powerinput/output Win(M), Wout(M) to/from main power storage device BA, upperlimits on electric power input/output Win(S), Wout(S) to/from selectedsub power storage device BB, total required power Pttl from total powercalculation unit 260, and a regenerative brake request made when thebrake pedal is operated. Traveling control unit 250 generates a controlmotor command, or torque command values Tqcom1 and Tqcom2, to allowmotor-generators MG1, MG2 to in total receive/output electric powerwithin a charging limit (Win(M)+Win(S)) and a discharging limit(Wout(M)+Wout(S)) in total for main power storage device BA and selectedsub power storage device BB. Furthermore, to ensure total required powerPttl, it is assigned between power provided by motor-generator MG2 todrive the vehicle and that provided by engine 4 to do so. In particular,externally charged battery's electric power is maximally utilized torestrict engine 4 from operation or the power provided by engine 4 todrive the vehicle is set to correspond to a range allowing engine 4 tobe highly efficiently operable to control the vehicle to travel toachieve high fuel-efficiency.

Inverter control unit 270 receives torque command value Tqcom1 and motorcurrent value MCRT1 of motor-generator MG1 and therefrom generatescontrol signals PWMI1, PWMC1 for inverter 14. Similarly, invertercontrol unit 280 receives torque command value Tqcom2 and motor currentvalue MCRT2 of motor-generator MG2 and therefrom generates controlsignals PWMI2, PWMC2 for inverter 22. Furthermore, traveling controlunit 250 generates a control engine command in response to a valuerequested of power provided by the engine to drive the vehicle, as set.Furthermore, a control device (an engine ECU) (not shown) controls theoperation of engine 4 in accordance with the control engine command.

Furthermore, traveling control unit 250 turns flag FLG on or off inresponse to operation of EV switch 40 by a user.

FIG. 6 is a flowchart for illustrating a flag FLG setting process bytraveling control unit 250. Control device 30 (traveling control unit250) can execute a previously stored, predetermined programperiodically, as predetermined, to repeatedly perform a controlprocessing procedure in accordance with the flowchart shown in FIG. 6,periodically as predetermined.

With reference to FIG. 6, traveling control unit 250 determines in stepS10 whether the travel mode is the EV mode or not. Traveling controlunit 250 determines, based on a result of operation of EV switch 40 by auser, whether the travel mode is the EV mode or not. Traveling controlunit 250 may also determine that the travel mode is the EV mode, when aprocess for generating engine control command is currently performed.

If it is determined that the travel mode is not the EV mode (NO in stepS10), then in step S11, traveling control unit 250 turns flag FLG(switching permission flag) off. That is, if the travel mode is the HVmode, flag FLG is set OFF, and therefore, switching of a selected subpower storage device is prohibited. On the other hand, if it isdetermined that the travel mode is the EV mode (NO in step S10), then instep S12, traveling control unit 250 turns flag FLG on. That is, if thetravel mode is the EV mode, flag FLG is set ON, and therefore, switchingof a selected sub power storage device is permitted.

Referring back to FIG. 5, when control device 30 (charging/dischargingcontrol unit 52) actively uses battery's electric power to travel (i.e.,in the EV mode) and total required power Pttl is equal to or smallerthan the batteries' total upper limit on electric power outputWout(M)+Wout(S), engine 4 is not operated but motor-generator MG2 aloneprovides power to drive the vehicle to travel. When total required powerPttl exceeds Wout(M)+Wout(S), engine 4 is started.

In contrast, when the EV mode is not selected, i.e., in the HV mode,control device 30 (charging/discharging control unit 52) controlsdistribution of driving power between engine 4 and motor-generator MG2to keep the batteries' SOC constant. In other words, traveling controlunder which travel with engine 4 is more actuatable than in the EV modeis carried out. Here, “batteries' SOC” refers to the SOC (remainingcapacity) value of the main power storage device and sub power storagedevices as a whole.

FIG. 7 is a flowchart for illustrating a charging/discharging controlprocess by charging/discharging control unit 52. Control device 30(charging/discharging control unit 52) can execute a previously stored,predetermined program periodically, as predetermined, to repeatedlyperform a control processing procedure in accordance with the flowchartshown in FIG. 7, periodically as predetermined. With reference to FIG.7, traveling control unit 250 determines in step S20 whether the travelmode is the EV mode or not. Note that as a method for determining thetravel mode, the method for determining in step S10 (FIG. 6) can beemployed. If it is determined that the travel mode is not the EV mode(NO in step S20), then in step S21, traveling control unit 250 controlscharging and discharging to and from the main power storage device and aselected sub power storage device such that the batteries' SOC is keptconstant. In other words, in the HV mode, charging/discharging controlunit 52 controls inverters 14, 22 and engine 4 in a manner maintainingthe batteries' SOC at a target value.

For example, charging/discharging control unit 52 controls, based onchange in the batteries' SOC, the amount of electricity generated bymotor-generator MG2 in braking of the vehicle, thereby controlling theamount of charge to the main power storage device and a selected subpower storage device. Alternatively, charging/discharging control unit52 controls, based on change in the batteries' SOC, distribution ofpower to drive the vehicle between engine 4 and motor-generator MG2,thereby controlling the amount of discharge from the main power storagedevice and a selected sub power storage device.

On the other hand, if it is determined that the travel mode is the EVmode (YES in step S20), then control for maintaining the batteries' SOCat a target value will not be carried out. In this case, in step S22,charging/discharging control unit 52 carries out a usualcharging/discharging control. In other words, charging/dischargingcontrol unit 52 controls inverters 14, 22 based on total required powerPttl, thereby controls charging and discharging to and from the mainpower storage device and a selected sub power storage device.

In the EV mode, charging and discharging are controlled topreferentially use the electric power of selected sub power storagedevice BB rather than that of main power storage device BA. As such,when the vehicle is traveling and currently used and selected sub powerstorage device BB is decreased in SOC, selected sub power storage deviceBB needs to be switched. For example, if battery BB1 is set as selectedsub power storage device BB in starting the vehicle, necessity willarise to subsequently disconnect battery BB1 from converter 12B andconnect battery BB2 as newly selected sub power storage device BB toconverter 12B, i.e., to perform a connection switching process.

Here, battery BB2 newly set as selected sub power storage device BB isgenerally higher in output voltage than battery BB1 that has been usedso far. Consequently, connection of a new high-voltage battery maycreate an unintended short-circuit path, which may give rise to aproblem in protection of equipment or the like. Therefore, in theprocess for switching connection of the sub power storage device,sufficient attention should be paid for preventing creation of ashort-circuit path. In addition, during a period for the connectionswitching process above, as electric power supply and electric powercollection by selected sub power storage device BB cannot be carriedout, charging and discharging should be restricted so as not to causeovercharge and overdischarge in the power supply system as a wholeduring that period.

The process for switching connection of the sub power storage devicewith attention being paid to such disadvantages will be describedhereinafter. Note that the “process for switching connection”corresponds to “switching control” described above.

FIG. 8 is a flowchart of a general procedure of the process forswitching a selected sub power storage device in the power supply systemof the electrically powered vehicle according to the embodiment of thepresent invention. Furthermore, FIGS. 9 to 12 are flowcharts forspecifically illustrating steps S100, S200, S300, S400, and S500 in FIG.7.

Control device 30 (switching control unit 51) can execute a previouslystored, predetermined program periodically, as predetermined, torepeatedly perform a control processing procedure in accordance with theflowcharts shown in FIGS. 7 to 12, periodically as predetermined. Theprocess for switching connection (switching control) of the sub powerstorage device in the power supply system of the electrically poweredvehicle according to the embodiment of the present invention can thus beimplemented.

With reference to FIG. 7, in step S100, switching control unit 51performs a process for determining switching of a selected sub powerstorage device. If switching control unit 51 determines that it isnecessary to switch the selected sub power storage device, the followingsteps S200 to S500 are performed. If switching control unit 51determines in step S100 that it is not necessary to switch the selectedsub power storage device, steps S200 to S500 are substantially notperformed.

In step S200, switching control unit 51 performs a pre-switching voltagestep-up process, and in step S300, performs an electric power limitmodification process so that a request is not generated to the powersupply system to excessively charge/discharge while connection of thesub power storage device is being switched. In step S400, control device30 performs the connection switching process for actually switchingconnection between selected sub power storage device BB and converter12B, and after completion of this process, in step S500, control device30 performs a return process to start electric power supply by newlyselected sub power storage device BB.

FIG. 9 is a flowchart for illustrating in detail the process in FIG. 8performed to determine whether the selected sub power storage deviceshould be switched or not (S100).

As will be described hereinafter, a variable ID is introduced toindicate the connection switching process's status. Variable ID is setto any of −1 and 0 to 4. ID=0 indicates a status in which no request forswitching a sub power storage device is generated. In other words, whenID=0, currently selected sub power storage device BB supplies electricpower, while whether selected sub power storage device BB should beswitched or not is determined periodically as predetermined. Meanwhile,when there is no sub power storage device that can newly be used due tofailure of equipment or consumption of electric power in the battery, itis assumed that ID=−1 is set.

With reference to FIG. 9, in step S105, switching control unit 51determines whether ID=0 or not. If ID=0 (YES in S105), in step S110,switching control unit 51 makes determination as to whether the selectedsub power storage device should be switched or not. Determination instep S110 is basically made based on a current SOC of selected sub powerstorage device BB. Namely, when the SOC of the sub power storage devicein use is lower than a predetermined criterion value, determination thatthe selected sub power storage device should be switched is made.

In step S150, switching control unit 51 checks a result of determinationin step S110 as to whether switching should be made or not. When it isdetermined that switching should be made (YES in step S150), switchingcontrol unit 51 determines in step S155 whether flag FLG (switchingpermission flag) is ON or not. If flag FLG is ON (YES in step S155),then switching control unit 51 designates in step S160, selected subpower storage device BB to newly be used. As shown in FIG. 1, in anexample where two batteries BB1 and BB2 are mounted as the sub powerstorage devices, newly selected sub power storage device BB isautomatically determined without the need to perform the processing instep S160. In the configuration in FIG. 1, however, if three or moreselected sub power storage devices BB1 to BBn (n is an integer notsmaller than 3) are mounted, a new sub power storage device to be usednext is designated based on an SOC or the like of each of the sub powerstorage devices that are not currently used. Then, switching controlunit 51 sets ID=1 in order to proceed with the connection switchingprocess. Namely, ID=1 indicates a status that a request for switchingselected sub power storage device BB is generated and the switchingprocess is started.

On the other hand, when it is determined in step S110 that switching ofthe selected sub power storage device is not necessary (NO in S150),switching control unit 51 maintains ID=0 in step S170. In addition, ifflag FLG is OFF (NO in step S155), then switching control unit 51 alsomaintains ID=0 in step S170. Meanwhile, when the switching process hasbeen started as relation of ID≧1 is once satisfied or when ID=−1 is setbecause there is no sub power storage device that can newly be used,processing in steps S110 to S180 is skipped.

FIG. 10 is a flowchart for illustrating in detail the pre-switchingvoltage step-up process (S200) shown in FIG. 8.

With reference to FIG. 10, in the pre-switching voltage step-up process,in step S205, switching control unit 51 confirms whether ID=1 or not. IfID=1, a request for switching selected sub power storage device BB isissued and the switching process is started (YES in S205), switchingcontrol unit 51 generates in step S210, a command to converter 12A tostep up voltage VH on electric power feeding line PL2 to a predeterminedvoltage V1. In response to the step-up voltage command, a voltagecommand value VHref for electric power feeding line PL2 is set to beequal to V1, and in order to implement this voltage command value,control signal PWUA for converter 12A is generated.

Note that predetermined voltage V1 is set to be higher than any higherone of respective output voltages of main power storage device BA andselected sub power storage device BB that is newly connected (forexample, BB2). For example, predetermined voltage V1 set at an upperlimit control voltage VHmax that can be stepped up by converter 12A canensure that voltage VH when a step-up voltage command is issued ishigher than both of the output voltages of main power storage device BAand selected sub power storage device BB after switching. Alternatively,in view of reducing a loss caused at converter 12A, predeterminedvoltage V1 may be determined, as occasion demands, to have a margin,depending on voltages output from main power storage device BA andselected sub power storage device BB after switching at that time.

If a step-up voltage command is generated in step S210, in step S220,switching control unit 51 determines based on a value detected byvoltage sensor 13 whether voltage VH has reached predetermined voltageV1 or not. Determination as YES is made in step S220, for example, whenVH≧V1 continues for a predetermined period of time.

Once voltage VH has reached predetermined voltage V1 (YES in S220),switching control unit 51 furthers the ID from 1 to 2. Until voltage VHreaches V1 (NO in S220), ID=1 is held. In other words, ID=2 indicates astatus in which the pre-switching voltage step-up process ends and theswitching process can be furthered. If ID≠1 (NO in S205), processing insubsequent steps S210 to S230 is skipped.

Thus, when the pre-switching voltage step-up process (step S200) ends,switching control unit 51 performs the electric power limit modificationprocess as shown in FIG. 11.

FIG. 11 is a flowchart for illustrating in detail the electric powerlimit modification process (S300) shown in FIG. 8.

With reference to FIG. 11, in the electric power limit modificationprocess, initially in step S305, switching control unit 51 determineswhether ID=2 or not. If ID=2 is not satisfied (NO in S305), processingin subsequent steps S310 to S340 is skipped.

If ID=2 (YES in S305), in step S310, switching control unit 51 startstemporary relaxation of charging and discharging restriction on mainpower storage device BA. Specifically, absolute values of upper limitson electric power input/output Win(M), Wout(M) to/from main powerstorage device BA are temporarily increased.

In addition, in step S320, switching control unit 51 gradually decreasesabsolute values of upper limits on electric power input/output Win(S),Wout(S) to/from selected sub power storage device BB. For example,Wout(S), Win(S) are decreased gradually toward 0 at a predeterminedfixed rate.

In step S330, switching control unit 51 determines whether Wout(S),Win(S) have reached 0 or not. Until Wout(S)=Win(S)=0, step S320 isrepeated to continuously decrease Wout(S) and Win(S).

Once Wout(S) and Win(S) have reached 0 (YES in S330), switching controlunit 51 furthers the ID from 2 to 3 in step S340. In other words, ID=3indicates a status in which the pre-switching voltage step-up processand the electric power limit modification process have ended andswitching of connection between sub power storage devices BB1, BB2 andconverter 12B can be started.

When the electric power limit modification process shown in FIG. 11ends, switching control unit 51 performs the process for switchingconnection of the sub power storage device in step S400.

FIG. 12 is a flowchart for illustrating in detail the process forswitching connection of the sub power storage device (S400), as shown inFIG. 8.

With reference to FIG. 12, in the process for switching connection ofthe sub power storage device, initially in step S405, switching controlunit 51 determines whether ID=3 or not. If ID≠3 (NO in S405), processingin subsequent steps S410 to S450 is skipped.

If ID=3 (YES in S405), in step S410, switching control unit 51 stopsconverter 12B to prepare for switching connection of the sub powerstorage device. More specifically, in converter 12B, IGBT devices Q1, Q2are forced to be turned off in response to a shutdown command, and inthat condition, switching control unit 51 generates in step S420 a relaycontrol signal for actually switching connection of the sub powerstorage device. For example, in order to disconnect battery BB1 fromconverter 12B and connect battery BB2 with converter 12B, relay controlsignals CONT4, CONT6 are generated to turn off relays SR1, SR1G, andrelay control signals CONT5, CONT7 are generated to turn on relays SR2,SR2G.

Furthermore, in step S430, switching control unit 51 determines whetheror not relay connection switching as instructed in step S420 has beencompleted. When the connection switching has been completed (YES inS430), switching control unit 51 restarts converter 12B to start aswitching operation in step S440, and furthers the ID from 3 to 4 instep S450.

In other words, ID=4 indicates a state in which switching of connectionbetween the sub power storage devices and converter 12B by means of therelays has been completed.

When the connection switching process in step S400 ends, switchingcontrol unit 51 performs the return process in step S500.

FIG. 13 is a flowchart for illustrating in detail the return process(S500) shown in FIG. 8.

With reference to FIG. 13, in the return process, switching control unit51 initially determines whether or not ID=4 in step S505. If ID≠4 (NO inS505), processes in subsequent steps S510 to S570 are skipped.

If ID=4 (YES in S505), in step S510, switching control unit 51 ends thetemporary relaxation of charging and discharging limits for main powerstorage device BA started in step S310 (FIG. 11). Thereby, Wout(M) andWin(M) basically return to values before the start of the switchingprocess for selected power storage device BB.

Further, switching control unit 51 gradually increases upper limits onelectric power input/output Win(S), Wout(S) to/from selected sub powerstorage device BB decreased to 0 in the electric power limitmodification process (step S300), to values of Win, Wout to/from a newlyselected sub power storage device (for example, battery BB2).

Then, in step S530, switching control unit 51 confirms whether or notupper limits on electric power input/output Win(S), Wout(S) havereturned to the values of Win, Wout to/from newly selected sub powerstorage device BB. During a period until return is completed (NO inS530), step S520 is repeatedly performed to gradually increase upperlimits on electric power input/output Win(S), Wout(S) at a fixed rate.

When return of upper limits on electric power input/output Win(S),Wout(S) is completed (YES in S530), switching control unit 51 returnsthe ID back to 0 in step S540. Thereby, a state in which normal supplyand recovery of electric power by main power storage device BA and newlyselected sub power storage device BB can be performed is reproduced inthe power supply system.

Further, the process proceeds to step S550 and switching control unit 51turns off the step-up voltage command generated in step S210 (FIG. 10).Thus, the voltage command value for electric power feeding line PL2 isalso set to an ordinary value set in accordance with the states of motorgenerators MG1, MG2.

After completion of a series of switching processes, switching controlunit 51 may further determine in step S560 whether or not there is apossibility that further switching of the selected sub power storagedevice is performed during traveling of the vehicle. If there is nopossibility of further switching, switching control unit 51 sets ID=−1in step S570. If ID=−1 is set, steps S100 to S500 in FIG. 8 aresubstantially not performed, and thus the switching process for theselected sub power storage device is not started until the vehicle stopsoperation.

On the other hand, if there is a possibility of further switching,switching control unit 51 skips step S570 and maintains ID=0. As aresult, the switching determination process in step S100 is performedperiodically as predetermined, and thereby the switching process for theselected sub power storage device is restarted as necessary.

Note that, in the exemplary configuration of FIG. 1 in which only twosub power storage devices are mounted, it is possible to omit theprocess in step S560, that is, always set ID=−1 once the switchingprocess for the selected sub power storage device is completed, therebylimiting the number of times the switching process for the selected subpower storage device is performed during driving of the vehicle, to onlyone.

Alternatively, in a power supply system equipped with three or more subpower storage devices or a power supply system having a configurationsuch that a sub power storage device not in use can be charged duringdriving of a vehicle, the power supply system can be configured suchthat a second or later switching process for a selected sub powerstorage device can be performed by maintaining ID=0 depending on asituation.

FIG. 14 shows an operation waveform diagram in the process for switchingthe selected sub power storage device in the power supply system of theelectrically powered vehicle according to the embodiment of the presentinvention described with reference to FIGS. 8 to 12.

With reference to FIG. 14, during a period until time t1 when ID=0, theswitching determination process is performed periodically aspredetermined, based on the SOC of the currently selected sub powerstorage device (e.g., battery BB1). Note that it is assumed that flagFLG in the ON state permits switching of a selected sub power storagedevice.

At time t1, in response to lowering in the SOC of battery BB1, theswitching determination process (step S100) is performed to issue arequest to switch selected sub power storage device BB and ID=1 is alsoset to start the switching process.

Thus, the pre-switching voltage step-up process (step S200) is performedand converter 12A increases voltage VH on electric power feeding linePL2 toward predetermined voltage V1. Processing for stepping up voltageon electric power feeding line PL2 is completed at time t2, andaccordingly, the ID is changed from 1 to 2.

When ID=2 is set, the electric power limit modification process (S300)is performed to temporarily relax charging and discharging to/from mainpower storage device BA. Namely, temporary increase in absolute valuesof upper limits on electric power input/output Win(M), Wout(M) isstarted. In addition, upper limits on electric power input/outputWin(S), Wout(S) to/from selected sub power storage device BB aredecreased toward 0 gradually at a fixed rate. It is noted that, duringthis period, converter 12B is controlled to stop charging/dischargingto/from the currently selected sub power storage device (battery BB1).Alternatively, converter 12B may be shut down from time t1.

At time t3, upper limits on electric power input/output Win(S), Wout(S)to/from selected sub power storage device BB are lowered to 0, and inresponse, the ID is changed from 2 to 3. Once ID=3 is set, the processfor switching connection of the sub power storage device starts. Morespecifically, with converter 12B being shut down, relays SR1, SR1G areturned off and thereafter relays SR2, SR2G are turned on. Then, when theprocess for switching connection by means of the relay is completed andbattery BB2 which is a newly selected sub power storage device isconnected to converter 12B, converter 12B is restarted. By completingthis connection switching process, the ID is changed from 3 to 4 at timet4.

When ID=4 is set, upper limits on electric power input/output Win(S),Wout(S) to/from selected sub power storage device BB are graduallyincreased at a fixed rate, so that use of battery BB2 which is a newlyselected sub power storage device is started. Accordingly, temporaryrelaxation of charging and discharging restriction on main power storagedevice BA is ended and Wout(M), Win(M) are basically caused to return tothe values at time t2 and before.

Then, when Win(S), Wout(S) of selected sub power storage device BBreturn to original values corresponding to Wout, Win of battery BB2 attime t5 respectively, return to ID=0 is made. Then, the processing forstepping up voltage on electric power feeding line PL2 is also stopped.

Thus, a series of processes for switching the selected sub power storagedevice ends and a state that normal electric power supply and electricpower collection with the use of selected sub power storage device BB(battery BB2) can be carried out is reproduced.

At time t5, when there is no possibility of the switching process as aresult of determination as to the possibility of the process for furtherswitching the sub power storage device during the operation of thevehicle as described in connection with FIG. 13, load subsequentlyimposed on control device 30 can be alleviated by setting ID=−1.

A configuration of switching control unit 51 will now be described usingFIG. 15. With reference to FIG. 15, switching control unit 51 includes aswitching determination unit 100, a step-up-voltage instruction unit110, electric power limiter units 120, 130, a connection control unit140, and a converter control unit 200. Step-up-voltage instruction unit110, electric power limiter units 120, 130, connection control unit 140,and converter control unit 200 constitute a “connection switching unit”of the present invention.

Switching determination unit 100 receives SOC(BB1), SOC(BB2) indicatingthe states of charge respectively of batteries BB1, BB2 and determineswhether the SOC of currently used selected sub power storage device BBis lower than a predetermined criterion value or not. When variable IDshared by the functional blocks is set to 0, switching determinationunit 100 performs the determination process above in predeterminedcycles.

When the selected sub power storage device should be switched and flagFLG is ON, switching determination unit 100 changes the ID from 0 to 1.Thus, a request for switching the selected sub power storage device isgenerated. In other words, switching determination unit 100 has afunction corresponding to the process in step S100 in FIG. 8.

When a request is generated to switch the selected sub power storagedevice and ID=1 is set, step-up-voltage instruction unit 110 outputs astep-up voltage command signal CMBT to converter control unit 200controlling converter 12A.

Converter control unit 200 generates control signals PWUA, PWDA forconverter 12A based on voltages VH, VLA and voltage command value VHref,so that voltage VH on electric power feeding line PL2 reaches voltagecommand value VHref.

Furthermore, when step-up-voltage instruction unit 110 generates step-upvoltage command signal CMBT, converter control unit 200 sets voltagecommand value VHref=V1 and generates control signal PWUA. If voltagesensor 13 detects voltage VH having reached predetermined voltage V1continuously for at least a predetermined period of time, convertercontrol unit 200 sets a flag FBT to ON indicating that stepping upvoltage is completed.

In response to flag FBT set to ON, step-up-voltage instruction unit 110sets ID=2 and continues to output step-up voltage command signal CMBTuntil a connection control unit 140, which will be described later,completes relay connection switching and ID=4 is set. In other words,step-up-voltage instruction unit 110 has a function corresponding tostep S200 in FIG. 8 and step S540 in FIG. 13.

An electric power limiter unit 120 sets upper limits on electric powerinput/output Win(S), Wout(S) to/from selected sub power storage deviceBB. Normally, upper limits on electric power input/output Win(S),Wout(S) are set based on selected sub power storage device BB orbattery's SOC (SOC(BB1) or SOC(BB2)), temperature (TBB1 or TBB2) and anoutput voltage (VB1 or VB2).

In the process for switching the selected sub power storage device, incontrast, when ID=2 is set, electric power limiter unit 120 decreasesupper limits on electric power input/output Win(S), Wout(S) gradually ata fixed rate toward 0, and when Win(S), Wout(S) have reached 0, electricpower limiter unit 120 changes the ID from 2 to 3. In addition, whenconnection control unit 140 sets ID=4, electric power limiter unit 120increases upper limits on electric power input/output Win(S), Wout(S) tovalues corresponding to Win, Win of newly selected sub power storagedevice BB after switching. Then, when the increase processing iscompleted, ID is changed from 4 to 0.

Namely, the function of electric power limiter unit 120 corresponds tothe processing in steps S320 to S340 in FIG. 11 and the processing insteps S520 to S540 in FIG. 13.

An electric power limiter unit 130 sets upper limits on electric powerinput/output Win(M), Wout(M) to/from main power storage device BA.Normally, upper limits on electric power input/output Win(M), Wout(M)are set based on main power storage device BA's SOC(BA), batterytemperature TA, and output voltage VA.

In contrast, during the process for switching the selected sub powerstorage device, when ID=2 is set, electric power limiter unit 130temporarily increases absolute values of upper limits on electric powerinput and output Win(M) and Wout(M), so as to temporarily relax chargingand discharging restriction on main power storage device BA. Then, whenconnection control unit 140 sets ID=4, electric power limiter unit 130causes upper limits on electric power input and output Win(M) andWout(M) to return to normal values.

Namely, the function of electric power limiter unit 130 corresponds tothe processing in step S310 in FIG. 11 and the processing in step S510in FIG. 13.

When electric power limiter unit 120 sets ID=3, connection control unit140 generates a command to shut down converter 12B, and also generatesrelay control signals CONT4 to CONT7 to switch connection betweenconverter 12B and sub power storage devices BB1, BB2. For example, whenselected sub power storage device BB is switched from battery BB1 tobattery BB2, relay control signals CONT4 to CONT7 are generated to turnoff relays SR1, SR1G and turn on relays SR2, SR2G. Once this relayconnection switching process is completed, connection control unit 140stops the shutdown command described above to restart converter 12B, andchanges the ID from 3 to 4.

The function of connection control unit 140 corresponds to the processin step S400 in FIG. 8 (the processes in S405 to S450 in FIG. 10).

In contrast, in the present embodiment, the switching control above isnot carried out in the HV mode. Furthermore, in the HV mode,distribution of driving power between engine 4 and motor-generator MG2is controlled to maintain the batteries' SOC at a target value.

FIG. 16 is a diagram illustrating control of batteries' SOC in the HVmode.

With reference to FIG. 16, prior to time t1, the travel mode ofelectrically powered vehicle 1 is the EV mode. In this case, althoughregenerative braking of motor-generator MG2 charges the main powerstorage device and a selected sub power storage device, basically,electric power is supplied from the main power storage device and theselected sub power storage device to motor-generator MG2 to causemotor-generator MG2 to generate power to drive the vehicle. Thus, thetotal SOC (batteries' SOC) lowers over time. When the HV mode isselected at time t1, traveling control unit 250 (see FIG. 5) maintainsthe total SOC at a target value A. This target value may be apredetermined value, or may be the SOC value at time t1, for example.

However, even if control for maintaining the total SOC value is carriedout, there may be possible lowering of the value for a reason such asthe vehicle's traveling condition. In this case, it is possible that theselected sub power storage device has an SOC value below a thresholdvalue used for determination as to whether switching should be made ornot.

FIG. 17 is a diagram illustrating a state in which the total SOC haslowered while electrically powered vehicle 1 is traveling in the HVmode. With reference to FIG. 17, when the HV mode is selected at timet11, traveling control unit 250 (see FIG. 5) seeks to maintain the totalSOC value at a predetermined target value (target value A shown in FIG.16). FIG. 17 shows a state in which the total SOC value graduallylowers. At time t12, the selected sub power storage device has an SOCvalue below a threshold value used for determination as to whetherswitching should be made or not.

It is assumed that at time t12, switching control of the selected subpower storage device is carried out. A spent sub power storage devicehas a low SOC value, and therefore, cannot be reconnected to converter12B when the vehicle is traveling. Accordingly, electric power stored inthe spent sub power storage device cannot be used for the vehicle totravel. As a result, the SOC value in total of the main power storagedevice and a plurality of sub power storage devices significantly(abruptly) drops at time t12.

Furthermore, during a period from time t12 to time t13, the total SOCvalue continues to lower. Meanwhile, at time t13, a user selects the EVmode. For example, the travel mode could be switched from the HV mode tothe EV mode when a user wishes to cause electrically powered vehicle 1to travel in the EV mode just before stopping electrically poweredvehicle 1. It is conceivable that such switching of a travel mode ismade for a reason such as reducing noise in residential areas late atnight or early in the morning and reducing exhaust gas in indoor parkinglots or garages.

However, since switching control of a selected sub power storage devicewas carried out at time t12, the total SOC value at the end of the HVmode (time t13) is far smaller than the value at the start of the HVmode (time t11). Therefore, a case occurs where despite the fact that auser has selected the EV mode, engine 4 operates so as to secure totalrequired power Pttl of the vehicle. In other words, the HV mode iscontinued. In this case, a user's needs can no longer be satisfied.

In contrast, in the present embodiment, even when, in the HV mode, aselected sub power storage device has a lower SOC value than a criterionvalue for determination as to whether switching should be made or not,switching control is not carried out. Therefore, even if the total SOCvalue continues to lower in the HV mode, a significant drop due toswitching of the selected sub power storage device can be avoided. Thus,the total SOC value continuously changes around time t12.

As a result, at the end of the HV mode (time t13), electric powernecessary for EV traveling can be ensured. At this time, even ifswitching control of the selected sub power storage device is carriedout because of the SOC of the selected sub power storage device below acriterion value for determination as to whether switching should be madeor not, a sub power storage device newly connected to converter 12Bstores sufficient electric power, which allows for EV traveling.

As above, in the present embodiment, connection of a plurality of subpower storage devices and an electric power feeding line is switchedwhen it is determined that a selected sub power storage device should beswitched. Switching of a selected sub power storage device is, however,prohibited when the travel mode is not the EV mode (i.e., when travelmode is the HV node). Charging and discharging to and from the mainpower storage device and a selected sub power storage device iscontrolled to keep constant value of state of charge of the main powerstorage device and a plurality of sub power storage devices when travelmode is the HV mode. When this control of charging and discharging isbeing carried out, switching of a selected sub power storage device isprohibited, even if the value indicating its state of charge lowers.Therefore, a significant (abrupt) drop of electric power available foran electrically powered vehicle caused by switching of a selected subpower storage device can be suppressed.

In the present embodiment, a configuration in which operation by a userswitches the EV mode and the HV mode has been illustrated. However, forexample, based on route information set by a navigation system, controldevice 30 may set a section for traveling in the EV mode and a sectionfor traveling in the HV mode and switch the EV mode and the HV modeaccording to the setting.

It should be understood that the embodiments disclosed herein areillustrative and non-restrictive in every respect. The scope of thepresent invention is defined by the terms of the claims, rather than thedescription above, and is intended to include any modifications withinthe scope and meaning equivalent to the terms of the claims.

The invention claimed is:
 1. A power supply system for an electricallypowered vehicle incorporating a motor for generating power to drive thevehicle and an internal combustion engine configured to be able togenerate said power to drive the vehicle independently of said motor,said electrically powered vehicle having, as a travel mode, a first modeallowing said motor to generate said power to drive the vehicle and asecond mode allowing said motor and said internal combustion engine togenerate said power to drive the vehicle, said power supply systemcomprising: a main power storage device for supplying electric power toan inverter for driving and controlling said motor; a plurality of subpower storage devices provided in parallel to each other; and aswitching control device configured to a selected sub power storagedevice for supplying electric power to said inverter, between saidplurality of sub power storage devices, said switching control deviceincluding: a switching determination unit for determining whether saidselected sub power storage device should be switched based on a state ofcharge of each of said plurality of sub power storage devices; and aconnection switching unit configured to switch said selected sub powerstorage device when said switching determination unit determines thatsaid selected sub power storage device should be switched, and saidswitching determination unit determining that said selected sub powerstorage device should not be switched regardless of said state of chargeof said selected sub power storage device, while said travel mode issaid second mode.
 2. The power supply system for an electrically poweredvehicle according to claim 1, wherein said electrically powered vehiclefurther includes a charging and discharging control unit for controllingcharging and discharging to and from said main power storage device andsaid plurality of sub power storage devices in accordance with saidtravel mode, and said switching determination unit receives a switchingprohibit instruction from said charging and discharging control unit. 3.The power supply system for an electrically powered vehicle according toclaim 2, wherein remaining capacity of said main power storage deviceand said plurality of sub power storage devices as a whole is controlledby said charging and discharging control unit to be kept constant insaid second mode.
 4. The power supply system for an electrically poweredvehicle according to claim 1, further comprising: an electric powerfeeding line for feeding electric power to said inverter; a firstvoltage converter provided between said electric power feeding line andsaid main power storage device, and configured to convert voltagetherebetween bidirectionally; a second voltage converter providedbetween said plurality of sub power storage devices and said electricpower feeding line, and configured to convert voltage between saidselected sub power storage device and said electric power feeding linebidirectionally; and a connection unit provided between said pluralityof sub power storage devices and said second voltage converter, andconfigured to selectively connect said selected sub power storage deviceto said second voltage converter.
 5. An electrically powered vehiclecomprising: a motor for generating power to drive the vehicle; aninternal combustion engine configured to be able to generate said powerto drive the vehicle independently of said motor; an inverter fordriving and controlling said motor; a main power storage device forsupplying electric power to said inverter; a plurality of sub powerstorage devices provided in parallel to each other; and a control deviceconfigured to switch a selected sub power storage device for supplyingelectric power to said inverter, between said plurality of sub powerstorage devices, said control device including: a switchingdetermination unit for determining whether said selected sub powerstorage device should be switched based on a state of charge of each ofsaid plurality of sub power storage devices; and a connection switchingunit configured to switch said selected sub power storage device whensaid switching determination unit determines that said selected subpower storage device should be switched, said electrically poweredvehicle having, as a travel mode, a first mode allowing said motor togenerate said power to drive the vehicle and a second mode allowing saidmotor and said internal combustion engine to generate said power todrive the vehicle, and said switching determination unit determiningthat said selected sub power storage device should not be switchedregardless of said state of charge of said selected sub power storagedevice, while said travel mode is said second mode.
 6. The electricallypowered vehicle according to claim 5, wherein said control devicefurther includes a charging and discharging control unit or controllingcharging and discharging to and from said main power storage device andsaid plurality of sub power storage devices in accordance with saidtravel mode, and said charging and discharging control unit generates aswitching prohibit instruction while said travel mode is said secondmode, and stops generation of said switching prohibit instruction whilesaid travel mode is said first mode.
 7. The electrically powered vehicleaccording to claim 6, wherein said charging and discharging control unitcontrols said inverter and said internal combustion engine such thatremaining capacity of said main power storage device and said pluralityof sub power storage devices as a whole is kept constant in said secondmode.
 8. The electrically powered vehicle according to claim 6, furthercomprising a travel mode setting device having a first state and asecond state corresponding to said first and second modes, respectively,either one of said first and second states being configured to bemanually settable, wherein said charging and discharging control unitsets said travel mode based on said one state in said travel modesetting device.
 9. The electrically powered vehicle according to claim5, further comprising: an electric power feeding line for feedingelectric power to said inverter; a first voltage converter providedbetween said electric power feeding line and said main power storagedevice, and configured to convert voltage therebetween bidirectionally;a second voltage converter provided between said plurality of sub powerstorage devices and said electric power feeding line, and configured toconvert voltage between said selected sub power storage device and saidelectric power feeding line bidirectionally; and a connection unitprovided between said plurality of sub power storage devices and saidsecond voltage converter, and configured to selectively connect saidselected sub power storage device to said second voltage converter. 10.A method for controlling an electrically powered vehicle, saidelectrically powered vehicle including: a motor for generating power todrive the vehicle; an internal combustion engine configured to be ableto generate said power to drive the vehicle independently of said motor;an inverter for driving and controlling said motor; a main power storagedevice for supplying electric power to said inverter; a plurality of subpower storage devices provided in parallel to each other; and a controldevice configured to switch a selected sub power storage device forsupplying electric power to said inverter, between said plurality of subpower storage devices, said method comprising the steps of: determiningwhether said selected sub power storage device should be switched basedon a state of charge of each of said plurality of sub power storagedevices; and switching said selected sub power storage device when saidstep of determining determines that said selected sub power storagedevice should be switched, said electrically powered vehicle having, asa travel mode, a first mode allowing said motor to generate said powerto drive the vehicle and a second mode allowing said motor and saidinternal combustion engine to generate said power to drive the vehicle,and wherein said step of determining determines that said selected subpower storage device should not be switched regardless of said state ofcharge of said selected sub power storage device, while said travel modeis said second mode.
 11. The method for controlling an electricallypowered vehicle according to claim 10, further comprising the steps of:setting said travel mode either in said first mode or in said secondmode; controlling charging and discharging to and from said main powerstorage device and said plurality of sub power storage devices inaccordance with said travel mode set by said step of setting; andprohibiting switching of said selected sub power storage device whensaid step of setting sets said travel mode in said second mode.
 12. Themethod for controlling an electrically powered vehicle according toclaim 11, wherein said step of controlling controls said inverter andsaid internal combustion engine such that remaining capacity of saidmain power storage device and said plurality of sub power storagedevices as a whole is kept constant in said second mode.
 13. The methodfor controlling an electrically powered vehicle according to claim 10,wherein said electrically powered vehicle further includes: an electricpower feeding line for feeding electric power to said inverter; a firstvoltage converter provided between said electric power feeding line andsaid main power storage device, and configured to convert voltagetherebetween bidirectionally; a second voltage converter providedbetween said plurality of sub power storage devices and said electricpower feeding line, and configured to convert voltage between saidselected sub power storage device and said electric power feeding linebidirectionally; and a connection unit provided between said pluralityof sub power storage devices and said second voltage converter, andconfigured to selectively connect said selected sub power storage deviceto said second voltage converter.